A metal and metal oxide measuring electrode of the above-mentioned kind is disclosed in published and examined German patent application DE-AS No. 21 21 047. In this electrode the metal is iridium, for example, and iridium oxide is the metal oxide; the part of the iridium electrode that is in contact with the fluid has a coating of iridium oxide. A method disclosed for producing this iridium oxide coating has three different steps for obtaining an oxide overlay that uniformly covers the iridium body.
From European patent application EP-PA No. 83108146.8, an electrochemical sensor for transcutaneous measurement of CO.sub.2 partial pressure is known, which again uses a measuring electrode containing iridium and iridium oxide. In this electrochemical sensor, the sensitive measuring part of the measuring electrode includes an iridium wafer that is electroplated onto a carrier plunger and at least partially oxidized by electrochemical and/or thermochemical oxidation.
It is characteristic of both known measuring electrodes that chemical, electrochemical or physical methods are used to produce the oxide layer, and that the oxide layer is accordingly very thin. As a result, the shape of the electrode is determined by the carrier of the oxide layer. Because of this, it is no longer possible to effect subsequent mechanical treatment of the measuring electrode in order, for example, to adapt it to geometric conditions at the location where it is installed after it has been mounted in a measuring head, or to clean off dirt deposits. Instead, in the known measuring electrodes, care must be taken that under no circumstances will they undergo any mechanical change or damage whatsoever once the oxide layer has been applied.